The present invention relates to an automatic equalization system for a line transmission system. It is well known that in a submarine cable system, the level diagram in the system fluctuates reflecting the change of the temperature at the sea-bottom, as the signal attenuation by a cable depends upon the temperature. Furthermore, the aging of cables and repeaters provide the fluctuation of the signal level in a cable. Most of theses level fluctuations have a frequency characteristics of the .sqroot.f type. The level variation or level fluctuation has various aspects. The level variation caused by the temperature change in a day has the repetition period of several hours, and has the seasonal level variation due to the seasonal temperature change. The level variation caused by the aging results in slow changes over life of the system. As the level variation in a cable system causes the degradation of the signal-to-noise ratio of the system, or overloads the repeaters in the system, a temperature equalizer is equipped in both the terminal stations of a cable system. Further, a pilot control type automatic equalizer is utilized for a submarine cable laid in shallow sea, for the purpose of the compensation of the short period level variations, in addition to the temperature equalizer for the seasonal level variations. However, a prior temperature equalizer for the seasonal level variation is a step-by-step equalizer which is adjusted by a maintenance engineer every time the pilot level variation exceeds a predetermined value. The maintenance operation such as above must be simplified while a higher precision of equalization is desirable. Consequently, the introduction of an automatic continuous variable type equalizer is desirable. Further, a prior pilot control type automatic equalizer is a so-called receiving terminal equalizer which equalizes the level variation within a cable system at the receiving terminal only. However, it should be appreciated of course that a pre- and post- equalization system which shares the equalization at both the transmission and receiving terminals is desirable in view of improving the signal-to-noise ratio.
One of the prior pre- and post- equalization systems has a memory for storing the operational level of the transmitting equalizer, and a calculator for calculating the difference between the stored level and the pilot level variation at the receiving terminal, and thus, the transmitting equalizer is controlled. However, said prior pre-equalizer has the disadvantages that the equipment is very large and uneconomical.